AD is backed up when you back up the system state of a DC. Even if the job is an incremental/differential job, the system state, and hence AD, would always be backed up as a full backup. The rest of the resources would backed up as an incremental/differential backup. For example, your incremental backup job backs up the C: drive as well as the system state, the system state would be backed up fully and the C: drive would be backed up incrementally.